Multiple wireless Tags can be interrogated by sending from an interrogating transmitter (e.g., a Reader) a code and having information transmitted by the Tag in response. This is commonly accomplished by having the Tag listen for an interrogation message and for it to respond with a unique serial number and/or other information. The Tags typically have limited power available for transmitting data wirelessly to the Reader. It is desirable to extend the range of wireless Tags so that it is not necessary to bring each Tag close to a Reader for reading.
Conventional RFID readers are typically designed to use one of three general approaches to transmit signals to and receive signals from one or more tag, including a single-channel homodyne technique, a two-antenna bi-static technique, and a technique to use a circulator device.
FIG. 4 shows one standard approach for an RFID reader to read a beam-powered tag in which a homodyne receiver is used. The term “homodyne receiver” refers to the fact that there is but a single channel for both the transmitted signal and the received signal and a direct down conversion of the data to baseband. This technique of a homodyne receiver is quite common and has the advantage of simplicity. It has the disadvantage of creating noise and lower sensitivity if it is not perfectly tuned. For example, the reader module 401 in FIG. 4 has an antenna 407 coupled to both a radio frequency (RF) source 403 and a receiver 405. Using the signal from the frequency source 403, the receiver 405 directly down converts the data received from the antenna 407.
FIG. 5 shows an approach of a bi-static design where a separate antenna is used for both transmit and receive. For example, a reader module 421 in FIG. 5 has a radio frequency source 423 coupled to its own antenna 427 to transmit signals and a receiver 425 coupled to its own antenna 429 to receive signals. This technique of bi-static design has at least two disadvantages. First, there is the added cost of two antennas instead of one. Since microwave antennas of some gain are expensive, the added cost of an additional antenna can be a major problem. Secondly, there is a requirement to have both antennas overlapping in pattern.
FIG. 6 shows a conventional technique to have a circulator in the reader to separate the incoming signal (to receive) from the outgoing signal (to transmit). A circulator couples the powers in a preferred direction so that the receiver retains backscatter information and the transmitter powers the tag. For example, the reader module 441 in FIG. 6 has a circulator 449 which couples power in a preferred direction, forward for transmit and power, and to the receiver for the receive or reflected portion. Power to the tag passes through to antenna 447 and received power from the tag is channeled toward the receiver block after being reflected by the tag. In FIG. 6, the circulator 449 couples port 2 to port 1 to transmit signals and couples port 2 to port 3 to receive signals. This approach also is quite common but has the disadvantage of requiring the circulator that is an expensive device.